Numerous approaches are being employed to improve the bone generation and repair cycle (also referred to as the bone repair cascade). Such issues are paramount in the treatment of all bone related defects related to degeneration, injury, infection, malignancy or developmental malformation. In the spinal surgery field, there are several different types of autologous bone graft substitutes that are either currently available or are in various stages of development for use in spine fusion surgery.
Demineralized bone matrix (DBM) is a manufactured product that has been readily available for over ten years. See for example, Grafton Putty (Osteotech, Eaton-town, N.J.); DBX Putty (MTF [Musculoskeletal Transplant Foundation], available through Synthes, Paoli, Pa.); and AlloMatrix Injectable Putty (Wright Medical Technology, Arlington, Tenn.). DBM is prepared by acid extraction of allograft bone, resulting in loss of most of the mineralized component but retention of collagen and noncollagenous proteins, including growth factors. DBM does not contain osteoprogenitor cells, but the efficacy of a demineralized bone matrix as a bone-graft substitute or extender may be influenced by a number of factors, including the sterilization process, the carrier, the total amount of bone morphogenetic protein (BMP) present, and the ratios of the different BMPs present (Strates et al., (1988) Am J Med Sci, 296:266-9; Urist et al., (1997) Connect Tissue Res, 36:9-20; and Sammarco and Chang, (2002) Foot Ankle Clin, 7:19-41). DBM includes demineralized pieces of cortical bone to expose the osteoinductive proteins contained in the matrix. These activated demineralized bone particles are usually added to a substrate or carrier (e.g. glycerol or a polymer). DBM is mostly an osteoinductive product, but lacks enough induction to be used on its own in challenging healing environments such as posterolateral spine fusion.
Allograft bone is a reasonable graft substitute for autologous bone. It is readily available from cadavers and avoids the surgical complications and patient morbidity associated with harvesting autologous bone. Allograft bone is essentially a load-bearing matrix comprised of cross-linked collagen, hydroxyapatite, and osteoinductive Bone Morphogenetic Proteins (BMP). Human allograft tissue is widely used in orthopaedic surgery. Allograft tissue is strong, integrates with the recipient host bone, and can be shaped either by the surgeon to fit the specific defect or shaped commercially by a manufacturing process. Allograft bone is available in two basic forms: cancellous and cortical. Cortical bone is a highly dense structure comprised of triple helix strands of collagen fiber reinforced with hydroxyapatite. The hydroxyapatite component is responsible for the high compressive strength and stiffness of bone while the collagen fiber component contributes to its elastic nature, as well as torsional, shear, and tensile strength. Cortical bone is the main load-bearing component of long bones in the human body.
Many devices of varying shapes and forms can be manufactured from cortical allograft tissue. Surgical implants such as pins, rods, screws, anchors, plates, and intervertebral spacers have all been made and used successfully in human surgery.
Even though allograft has certain advantages over the other treatments, one of the main drawbacks of the allograft treatment is that the ingrowth of the host bone into the grafted bone may take longer than in an autograft. As a result, allograft treatment may be less effective than the autograft. Attempts have been made to overcome these drawbacks by modifying the bone graft's surface. For example, U.S. Pat. No. 6,511,509 discloses a textured graft, wherein the texturing comprises a plurality of closely spaced continuous or discrete protrusions.
U.S. Pat. No. 6,458,168 teaches a graft comprising a combination of two cortical bone portions and a cancellous bone portion located between the cortical bone portions. According to the disclosure, the portions of the composite graft are held together by means other than adhesive and not demineralized.
U.S. Pat. No. 6,899,107 discloses a graft coated with a biopolymer seeded with periosteal cells harvested from either the graft recipient or from an allogenic or a xenogenic source.
U.S. Patent Application Publication No. 20040228899 teaches the use of bone grafts, including allografts, characterized by tartrate-resistant acid phosphatase (TRAP) adsorbed to a porous hydroxyapatite substratum.
Despite the advances recently made in the art, new methods promoting ingrowth of the host bone into the grafted bone are needed to better utilize the advantages of allograft treatment.